The rapid squirt of a proteinaceous slime jet endows velvet worms (Onychophora) with a unique mechanism for defence from predators and for capturing prey by entangling them in a disordered web that immobilizes their target. However, to date, neither qualitative nor quantitative descriptions have been provided for this unique adaptation. Prof. L. Mahadevan and researchers from universities in Chile, Costa Rica, and Brazil report their investigation into the fast oscillatory motion of the oral papillae and the exiting liquid jet that oscillates with frequencies f~30–60 Hz. Using anatomical images, high-speed videography, theoretical analysis and a physical simulacrum, the scientists show that this fast oscillatory motion is the result of an elastohydrodynamic instability driven by the interplay between the elasticity of oral papillae and the fast unsteady flow during squirting. Their results demonstrate how passive strategies can be cleverly harnessed by organisms, while suggesting future oscillating microfluidic devices, as well as novel ways for micro and nanofibre production using bioinspired strategies.